Use of envelope following techniques for linear modulation is highly desirable for long term evolution (LTE) customers and others in the years to come, because envelope following and pseudo-envelope following enables a very efficient use of energy. However, using envelope following techniques adds production line calibration requirements that may become a burden to phone manufacturers due to excessive calibration time overhead. For the purpose of this disclosure, envelope following systems include pseudo-envelope following systems, wherein pseudo-envelope following is envelope tracking that includes power amplifier (PA) collector/drain voltage pre-distortion to ameliorate power amplifier nonlinearity. It should be understood that envelope following is sometimes referred to as envelope tracking by some.
Envelope following systems are power management systems that control power amplifiers (PAs) in such a way that PA collector/drain voltage (referred to herein as Vcc) follows an RF input signal envelope, which is an instantaneous voltage of a PA input RF signal, (referred to herein as VIN). Implementing pseudo-envelope following improves overall efficiency of PA systems because a power management function is realized using high efficiency switcher systems. However, a modulation of collector voltage results in some detrimental side effects due to the physics of transistors. One such detrimental side effect is gain modulation. For example, PA gain depends on Vcc, thus modulating PA collector also voltage modulates PA gain that creates a significant nonlinearity.
Traditionally, the problem of nonlinearity is solved in pseudo-envelope following systems by following a supply voltage/input power (Vcc/PIN) curve for which gain remains constant. The Vcc/PIN curve is known as an isogain curve and a related Vcc/VIN rule is programmed into a special look-up table (LUT) that is included in a phone's transceiver circuitry. What is needed is fast calibration methods that use the phone's transceiver circuitry to relatively quickly complete a calibration procedure that produces results that are usable to generate the special LUT.